Such methods for ascertaining and monitoring fill level in a container are frequently applied in measuring devices of automation and process control technology. Such fill level measuring devices are produced and sold by the assignee, for example, under the marks, Prosonic, Levelfiex and Micropilot. These work according to the travel time-measuring method and serve to determine and/or to monitor a fill level of a medium in a container. These fill level measuring devices transmit by means of a transmitting/receiving element a periodic transmission signal, in the form of microwaves or ultrasound, toward the surface of a fill substance, and receive the reflected echo signals after a distance-dependent travel time. Usual, commercially available fill level measuring devices working with microwaves can basically be divided into two classes; a first class, in the case of which the microwaves are transmitted by means of an antenna toward the fill substance, reflected off the surface of the fill substance, and then, after a distance-dependent travel time, are received back; and a second class, in the case of which the microwaves are conveyed along a waveguide toward the fill substance, are reflected off the surface of the fill substance due to an impedance jump existing there, and the reflected waves are led back along the waveguide.
The travel time measuring method can essentially be subdivided into two methods: The first method is based on a travel time measurement, which requires a pulse sequence modulated signal for the traveled path; a second widespread method is based on determining the frequency difference of the currently transmitted, continuously frequency-modulated high-frequency signal relative to the received, reflected high-frequency signal (FMCW—Frequency-Modulated Continuous Wave). Generally, in the following explanations, no limitation is made to a particular one of these methods.
From the received echo signals, there is formed, as a rule, an echo function representing the echo amplitudes as a function of travel time, wherein each value of this echo function corresponds to the amplitude of an echo reflected at a particular distance from the transmission element.
In this ascertained echo function, a wanted echo is determined, which corresponds to the reflection of the transmission signal off the surface of the fill substance. From the travel time of the wanted echo, there is directly obtained, in the case of known propagation velocity for the transmission signals, the distance between the surface of the fill substance and the transmission element.
In order to simplify the echo curve evaluation, the raw received signals of the pulse sequence are not used, but, instead, the envelope, the so called envelope curve, is ascertained. The envelope curve is acquired, for example, by rectifying the raw signal of the pulse sequence and then filtering via a lowpass.
There are a number of different methods for determining the wanted echo in an envelope curve.
In a first method, the wanted echo, which has a larger amplitude than the remaining echos, is selected by a static echo search algorithm. Thus, the echo in the envelope curve with the largest amplitude is ascertained as the wanted echo.
In a second method, it is assumed in a static echo search algorithm that the wanted echo is the first echo occurring in the envelope curve after the transmission pulse. Thus, the first echo in the envelope curve is selected as the wanted echo.
It is possible to combine these two methods with one another, in that, for example, a so-called first echo factor is defined. The first echo factor is a predetermined factor, by which an echo must exceed a particular amplitude, in order to be recognized as the wanted echo. Alternatively, a travel time-dependent echo threshold can be defined, which an echo must exceed, in order to be recognized as the wanted echo.
In a third method, the fill-level measuring device is informed once of the actual fill level. The fill level measuring device can, on the basis of this experience, identify the associated echo as the wanted echo, and, for example, follow it with a suitable dynamic echo search algorithm. Such methods are referred to as echo tracking. In such case, in, for example, each measuring cycle, maxima for the echo signal or the echo function are determined, and, based on the knowledge of the fill level ascertained in the preceding measuring cycle and an application-specific maximum rate of change to be expected for the fill level, the wanted echo is ascertained. From a travel time of the so ascertained, current wanted echo, the new fill level is then obtained.
A fourth method is described in DE 102 60 962 A1. There, the wanted echo is ascertained on the basis of data stored earlier in a memory. In such case, from received echo signals, echo functions are derived, which reflect the amplitudes of the echo signals as a function of their travel time. The echo functions are stored in a table, wherein each column in each case serves for recording an echo function. The echo functions are stored in the columns in a sequence, which corresponds to the fill levels associated with the particular echo functions. During operation, the wanted echo and the associated fill level are determined on the basis of the echo function of the current transmission signal with the assistance of the table.
In DE 103 60 710 A1, a fifth method is described, in the case of which, transmission signals are periodically transmitted toward the fill substance, their echo signals recorded and converted into an echo function, at least one echo characteristic of the echo function is determined, and, on the basis of the echo characteristics of at least one preceding measuring, a prediction for the echo characteristics to be expected in the case of the current measuring is derived. Taking into consideration the prediction, the echo characteristics of the current measuring are determined, and, on the basis of the echo characteristics, the current fill level is ascertained. This method approaches an echo tracking in the broadest sense.
In DE 10 2004 052 110 A1, a sixth method is described, which achieves improvement of the wanted echo detection by an echo evaluation and classification of the echos in the envelope curve.
Corresponding to the state of the art set-forth above, there are different approaches to determine the exact position of the fill level wanted/echo signal in the ascertained echo curve or in the digitized envelope curve. The accuracy of measurement that can be achieved with this echo measuring principle under the given measuring conditions depends, however, on what measurement accuracy can be achieved with this echo measuring principle under the given measuring conditions. Taken by themselves, these methods described above in each case work without problem in a large number of applications. Problems always occur, however, when the echo stemming from the fill level cannot be identified on the basis of the method without there being some doubt as to the correctness of the identification.
In the case of the first method, for example, measurement problems occur, if installed objects are present in the container, which reflect the transmission signals better than the surface of the fill substance.
In the case of the echo tracking according to the third method, measurement problems occur if, during operation, the wanted echo overlaps a disturbance echo, and the disturbance echo is subsequently tracked as a wrong wanted echo. Furthermore, a problem occurs if, when the device is turned on, the previous wanted echo signal no longer agrees with the current one, or the previous wanted echo signal is not known.
If another echo than the fill-level echo is mistakenly classified as the wanted echo, the danger exists that a wrong fill level is output, without this being noticed. This can, depending on application, lead to an overfilling of containers, to pumps running empty or to other events in given cases connected with considerable dangers.